Because of transmission distortion, a transmission loss, and the like, long-distance transmission is usually unsuitable for a baseband signal on various channels. To enable long-distance transmission of the baseband signal on various channels, corresponding carrier modulation needs to be performed on the baseband signal, so that the baseband signal is transmitted on a signal spectrum of a high frequency, and the baseband signal becomes a signal suitable for long-distance channel transmission.
Specifically, carrier modulation may be currently performed in a digital modulation manner such as Quadrature Phase Shift Keying (QPSK) or Quadrature Amplitude Modulation (16QAM). In addition, with successive commercial use of the QPSK and the 16QAM, and a continuously increasing transmission capacity requirement, signal modulation starts to be performed by using higher-order QAM. However, anti-noise performance of the higher-order QAM is far poorer than that of the QPSK and that of the 16QAM. Therefore, when quantization noise is relatively great because a currently commercial high-speed Digital-to-Analog Converter (DAC) has a relatively small ENOB (Effective Number of Bits, effective number of bits), back-to-back BER (Bit Error Rate, bit error rate) performance of the higher-order QAM is quite poor, and a signal basically cannot be transmitted. Consequently, system reliability of the higher-order QAM is relatively low.
To improve the system reliability of the higher-order QAM, currently, a signal is usually processed and transmitted by using transmitters of the following two structures in the industry:
Transmitter 1: As shown in FIG. 1, the transmitter may include a light source, a multicarrier generation apparatus, a demultiplexer, a polarization multiplexing In-phase Quadrature (IQ) modulator, a coupler, and the like. A working principle of the transmitter is as follows: The light source generates a consecutive optical carrier with a specified wavelength, the multicarrier generation apparatus converts the consecutive optical carrier into optical carriers with a plurality of wavelengths, the demultiplexer demultiplexes the optical carriers to generate a plurality of subcarriers, the polarization multiplexing IQ modulator performs polarization multiplexing IQ modulation on each subcarrier based on a plurality of low-speed baseband IQ signals obtained by performing corresponding conversion on a high-speed data signal, and the coupler combines and outputs all modulation subcarrier signals.
That is, a data signal with a high Baud rate may be changed into a plurality of data signals with a low Baud rate in a manner of multiplexing a plurality of subcarriers, so that only a low-bandwidth device (for example, a low-bandwidth DAC and another low-bandwidth electrical device) needs to process and transmit a corresponding data signal. The low-bandwidth DAC usually obtains a relatively high ENOB in a relatively easy manner, and therefore has relatively small quantization noise. In addition, the another low-bandwidth electrical device usually has relatively small electrical noise. Therefore, this manner can improve system reliability of the higher-order QAM. However, the multicarrier generation apparatus and the demultiplexer that are required in this manner can still be implemented only in a relatively complex and high-cost manner. Therefore, this manner cannot really achieve a commercial purpose because of problems such as complex implementation and high costs.
Transmitter 2: As shown in FIG. 2, the transmitter may include a pulsed light source, a Differential Phase Shift Keying (DPSK) encoder, a phase modulator, a spectral phase encoder, and the like. A working principle of the transmitter is as follows: The DPSK encoder generates a differentially coded signal according to an input signal, and outputs the differentially coded signal to the phase modulator, the phase modulator performs, according to the differentially coded signal, phase modulation on an optical carrier generated by the pulsed light source, and outputs a modulation signal to the spectral phase encoder, and the spectral phase encoder performs phase modulation on the modulation signal to implement spectrum spreading. Specifically, when performing phase modulation, the spectral phase encoder needs to divide the signal into a plurality of spectral components, and perform phase modulation on each spectral component according to different optical phase shifts. Each optical phase shift is usually generated by a plurality of delay units, or generated by a controllable phase unit including lithium niobate or other electro-optic material equivalent to lithium niobate.
That is, the spectral phase encoder may perform phase modulation on the modulation signal, so as to implement spectrum spreading, and improve QAM system reliability. However, in this method, a plurality of delay units or controllable phase units are usually required to generate a corresponding optical phase shift, so as to implement spectrum spreading. Consequently, this method cannot really achieve a commercial purpose because of relatively complex implementation and relatively high costs.
In conclusion, problems such as relatively high costs and a relatively great difficulty in implementation exist in an existing manner of improving QAM system reliability. Therefore, a new manner is urgently required to resolve the foregoing problems.